1. Field of the Invention
The present invention relates to a semiconductor light emitting device, particularly to improvement of the electrode structure of a semiconductor chip constituting a semiconductor light emitting device.
2. Description of the Prior Art
A semiconductor light emitting device is generally configured by using solder to anchor an electrode, formed on the top of the stack structure of a semiconductor chip obtained by processing a stack structure comprising a semiconductor substrate and a p-n junction formed on the semiconductor substrate by epitaxial growth, to a metallic stem or mount constituting an electrode of a semiconductor device. The configuration is known as mounting in the junction-down style, because the stack layer structure side is anchored to the stem.
The electrode formed on the top of the stack structure of the semiconductor chip provides an electrode for the p-type region when the semiconductor substrate is of n-type, while, when the semiconductor substrate is of p-type, it provides an electrode for the n-type region. In either case, it generally has a stack structure comprising a metallic layer which provides ohmic contact with a p-type or n-type semiconductor and a metallic layer which is anchored to the stem.
The semiconductor chips are divided into generally square or rectangular individual chips in the state where they have the electrode metallic layer on the top, after completion of the processing including the semiconductor layer stacking process (generally known as wafer process) where the geometry of the semiconductor substrate is maintained, and further passed through processing (generally known as the assembling process, including a dividing process) before being completed as a semiconductor device. The dividing operation is performed from the top side of the electrode metallic layer by using a cutting tool such as a diamond saw, known as a dicer, for such a device as a light emitting diode, and for a semiconductor laser, by using a diamond stylus with an apparatus called a scriber, to draw marking-off lines (also called scribe lines) in a specified direction with a specified spacing, and then making cleavage of the semiconductor substrate along the marking-off line. For a semiconductor laser, the cleavage is made two times, the first cleavage (known as the primary cleavage) producing small long bars, with one of the opposed cleavage surfaces being coated with a high reflectivity film, while the other is coated with a low reflectivity film, then the secondary cleavage providing individual chips.
Therefore, for a semiconductor light emitting device, the operation of anchoring to the stem is performed in the state where at least one of the two opposed sides of the semiconductor chip is exposed, in other words, the p-n junction is exposed. The anchoring is performed by compression bonding the semiconductor chip to the metallic stem with solder, such as indium (In), being interposed between the electrode layer on the semiconductor chip side and the surface of the metallic stem, and heating and melting the solder to alloy it with the electrode layer.
Thus, the metal constituting the electrode layer on the side whereon the semiconductor chip is anchored to the stem must (1) be soft enough to allow it to be compression bonded to the stem through the solder, i.e., have a sufficiently high ductility, and (2) be able to be alloyed with the solder at a low temperature which will not deteriorate the electrical and optical characteristics of the semiconductor chip, and so gold (Au) is generally used.
After the semiconductor chip has been anchored, the solder may often irregularly spread along the side from the anchoring surface with the end of the spread solder extending linearly (which is generally called creep).
When the creep of the solder is caused on the side where the p-n junction is exposed, and the end of the creep reaches the p-n junction, the p-n junction is short-circuited. A semiconductor light emitting device is forward biased for operation, which means that short-circuiting of the p-n junction will not immediately cause the semiconductor light emitting device to be inoperative. However, because a part of the forward bias current to contribute to light emission flows through the solder which has crept, the ratio of the amount of light emission to the current flowing through the device, i.e., the light emission efficiency is lowered. Further, with a semiconductor laser, the laser oscillation threshold current is increased. In addition, if the p-n junction is short-circuited, the reverse field dielectric strength characteristic and various other electrical characteristics of the semiconductor light emitting device are deteriorated. When the values predetermined for the characteristics of a semiconductor light emitting device are not met, resulting from the deterioration of the light emission efficiency and the various electrical characteristics, and the increase of the laser oscillation threshold current, the semiconductor light emitting device is judged to be a defective item. In addition, for a semiconductor light emitting device which has been judged as only just acceptable, being in the state extremely close to the limits of the values, the probability of it offering unsatisfactory characteristics in a relatively short time is extremely high.
Thus, creep of the solder on the side of a semiconductor chip can reduce the yield (non-defective percentage) and the reliability of a semiconductor light emitting device.
As a configuration intended to prevent the reduction in yield and reliability due to creep of the solder, Sugo et al. (Japanese Unexamined Patent Publication No. 8(1996)-172238), for example, has disclosed a method of manufacturing a semiconductor laser device which, in the wafer process, provides a groove of approx. 3 .mu.m in depth reaching the semiconductor substrate in an area where a scribe line for making the secondary cleavage would be drawn, and covers the sides and bottom of the groove with an insulating film. With the semiconductor laser chip according to this configuration, one of the two sets of opposed sides are coated with a high reflectivity film and a low reflectivity film as described above, and the remaining one is coated with an insulating film, thus, the p-n junction is not exposed on any sides, so if creep of the solder is caused in chip anchoring, the p-n junction will not be short-circuited, and thus reduction in yield and reliability of a semiconductor laser device will not occur.
By the way, the stack structure of a semiconductor laser device and other semiconductor light emitting devices comprises a few types of semiconductor layers which are generally different in composition from one another. When the groove is provided in the stack structure by chemical etching, the etching rate for a given etchant is not always uniform for semiconductors having different compositions, which means that the amount of etching varies for the layers, resulting in irregularities being produced on the sides of the groove provided. Especially when the stack structure includes an InGaP base layer and a GaAs or AlGaAs base layer, an etchant for GaAs or AlGaAs base layers cannot etch an InGaP base layer, which means a plurality of etchants must be used in the process of providing the groove, which results in the process being complicated, and in some cases, the irregularities on the sides of the groove being heavier. It is extremely difficult to completely cover the sides and bottom of the groove having irregularities on the sides with the insulating film. Especially for the areas which are invisible when viewed from the opening of the groove, it is practically impossible to cover them when the irregularities are heavy.
Use of a physical etching method, such as ion sputtering, can solve the above problem. However, physical etching is carried out at an extremely low speed, as compared to that for chemical etching, thus, to produce the groove having a depth of approx. 3 .mu.m and a width of 100 .mu.m or so to allow inserting the tip of a diamond stylus in the wafer surface as many times as required in parallel with a set spacing, it takes a long time. In other words, physical etching is feasible, but not easy to realize.
That is to say, the above structure offers the possibility of preventing the reduction in yield and reliability of a semiconductor light emitting device due to a creep of the solder, but is not easy to realize.
In the current situation wherein the semiconductor light emitting device is widely used in large quantities not only in industrial applications, but also in public welfare applications, there is a strong demand for reduction of the manufacturing cost and improvement of the reliability. Therefore, a highly feasible technique which can prevent short-circuiting of the p-n junction due to a creep of the solder, and thus can prevent the reduction in yield and reliability of a semiconductor light emitting device is very much in demand.
One of the reasons why the Au film is generally used as an electrode metallic layer on the side whereon the semiconductor chip is anchored to the stem is that it has an extremely high ductility, as stated above. Therefore, the Au film can easily be spread in cutting with a diamond saw or marking-off and cleavage with a diamond stylus in the dividing operation. Thus, after the dividing operation, the spread Au film may often become stuck to the side of the semiconductor chip or protrude as a flash from the side. Further, it has been already confirmed that, in anchoring of the semiconductor chip, the solder creeps, starting at a place where the Au film sticks to the chip side or protrudes as a flash from the side.
The above problems cannot be solved if a metallic film other than Au is selected as the electrode metallic layer on the side whereon the semiconductor chip is anchored to the stem. This is because, resulting from the high ductility, which is the primary criterion in selecting the electrode metallic layer on the side on which the semiconductor chip is anchored to the stem, it is not possible to avoid the electrode film sticking to the side of the semiconductor chip or protruding as a flash from the side in the dividing operation, and further, low temperature alloying of the electrode film with the solder, which is the secondary criterion for selection, causes the solder to creep, starting at a place where the electrode film sticks to the side or protrudes as a flash from the side.
The semiconductor light emitting device according to the present invention comprises a semiconductor stack portion wherein a plurality of semiconductor layers are stacked; a second metallic layer which contacts at least a part of the top of the semiconductor stack portion; and a first metallic layer which contacts the second metallic layer; wherein at least a part of the outer edge of the first metallic layer is located inside the outer edge of the second metallic layer, and an area where the top of the second metallic layer is exposed is provided outside the outer edge of the first metallic layer.
It is preferable that the second metallic layer comprise a high-melting point metal, or be made by stacking a plurality of metallic layers, and at least the topmost layer of the stack metal layer is a high-melting point metallic layer.
It is preferable that the high-melting point metal be any one of platinum (Pt), titanium (Ti), molybdenum (Mo), and tungsten (W).
The effects of the present invention will be described with an Au film being used as a typical example of an electrode metallic layer on the side on which the semiconductor chip is anchored to the stem.
According to the structures of the present invention, an area where the Au film does not exist can be provided along the plane tangent to, at least, the side where the p-n junction is exposed, of the semiconductor chip for a semiconductor light emitting device. Therefore, in dividing of semiconductor chips, the diamond stylus or diamond saw will not touch the Au film, and thus the Au film will not be spread. Thus, after the dividing operation, sticking of the Au layer to the side where the p-n junction is exposed or protruding of it as a flash from the side is not observed, and in anchoring, creep of the solder starting at a place where the Au film sticks to the side or protrudes as a flash from the side will not be caused.
Further, according to the structures of the present invention, the Au film is provided contacting a part of the top of the second metallic layer contacting the top of the semiconductor stack portion, so that in the areas where the Au film is not applied, the top of the second metallic layer is exposed, which means that deterioration of the characteristics of a semiconductor light emitting device due to the solder coming into direct contact with the semiconductor stack top to heat it when anchoring the semiconductor chip will not be caused.
Partial removal of the Au layer can be easily achieved by using the conventional technique which is based on photolithography and chemical etching.
With the structures of the present invention, the second metallic layer will be reliably cut with a diamond saw or a diamond stylus in a dividing operation. Therefore, if the second metallic layer comprises a metal which is lower in ductility and less reactive in alloying with the solder than the Au film, there remains the possibility of short-circuiting of the p-n junction due to creep of the solder.
The above problem can be avoided by using, as the second metallic layer, a high-melting point metal which will not cause alloying reaction with the solder at the heating temperature in anchoring.
In addition, the metal which provides ohmic contact with a p-type or n-type semiconductor is not always a high-melting point metal or a metal having a high strength of bond to the Au film. However, by giving the second metallic layer a two-layer stack structure comprising a metallic layer which provides ohmic contact with the semiconductor and a high-melting point metal having a high strength of bond to both of the metallic layer and the Au film, creep of the solder can be prevented without causing peeling of the Au film.
When there is a need for the second metallic layer to have a tack structure of three layers or more to relieve the high internal stresses imposed on the high-melting point metallic layer and prevent peeling of the high-melting point metallic layer due to the high internal stresses, using, as the layer contacting the semiconductor, a metallic layer providing ohmic contact with the semiconductor, and a high-melting point metallic layer as the topmost layer can prevent creep of the solder in the anchoring operation.
As a high-melting point metal which will not cause an alloying reaction with the solder at the heating temperature in anchoring, platinum (Pt), titanium (Ti), molybdenum (Mo), and tungsten (W) are available.